The effects of geometric parameters under small and large deformations on dissipative performance of shape memory alloyhelical springs

This paper presents an investigation into shape memory alloy (SMA) springs considering the effects ofgeometry changes under small as well as large deformations. Helical springs are fabricated by shape setting of NiTiwires through heat treatment followed by cooling in furnace. The products exhibit pseudoelasticity at the ambienttemperature, and their force-displacement responses are examined by conducting simple tension test. A model isfurther proposed to study tension and compression of SMA springs, and it is shown that the consequences ofgeometrical changes in tension and compression of springs are different. Under tension, at early stages of loading, theassumption of small deformations can reasonably predict the response of an SMA spring; however, the error of usingsmall deformation assumption increases with increase in the amount of applied force. Unlike tension, in compression,small deformation model predicts the response of SMA spring with a considerable error even from the beginning ofloading. The numerical results of large and small deformation models are verified by experimental findings. In order todesign a spring having maximum dissipative performance, a designer has three geometric parameters to set: wirediameter, spring diameter and the number of active coils. The influences of these parameters on dissipated energy areinvestigated in both displacement- and force-control loadings, and a framework for designing SMA springs with thepurpose of achieving maximum applicable dissipation is at last developed.

Keywords:

shape memory alloy , SMA , spring , large deformation , dissipated energy , geometric parameters